In EMFC weighing cells, the weight of the load is transmitted either directly or by way of one or more force-transmission levers to an electromechanical measurement transducer which delivers a signal corresponding to the weighing load. The signal is further processed by an electronic portion of the weighing instrument and the result is presented on a display.
Weighing cells with a strain transducer contain a deformable body which is provided with strain gauges. Placing the load on the weighing cell causes an elastic deformation of the deformable body. In many cases, the deformable body is configured as a parallelogram-shaped measuring element, specifically as a parallel-guiding mechanism with specially designed bending zones, whereby defined zones of deformation are created where the strain gauges are arranged. As a result of the load placed on the movable parallel leg, the strain gauges are subjected to tension or compression which causes a change of their electrical resistance in comparison to the load-free state of the movable parallel leg, wherein the change in resistance represents a measure for the applied load.
In string-oscillator weighing cells the mechanical design structure is largely analogous to EMFC- and strain gauge weighing cells, except that an oscillating-string transducer is used in place of an electromagnetic measurement transducer. As a result of the load, the tension in an oscillating string is increased, and the frequency change, in turn, represents a measure for the applied load.
The weighing cells of the foregoing description share an essential trait which is common to all gravimetric measuring instruments with parallel-guided weighing pans, namely that the weight force transmitted from the weighing pan to the measurement transducer in general depends to a small degree on whether the weighing load is placed on the center of the weighing pan or is shifted out of the center towards the periphery of the weighing pan. This can have the undesirable consequence that a balance indicates different amounts of weight for one and the same weighing load, depending on where the weighing load was placed on the weighing pan. These deviations which are caused by an eccentric placement of the weighing load on the weighing pan are commonly referred to as corner load errors.
In a parallelogram-shaped measuring element or a parallel-guiding mechanism which constrains the weighing pan carrier to a parallel movement by means of two parallel, essentially horizontal parallel guides, corner load errors are caused primarily by the fact that the parallel guides deviate slightly from an ideal, absolutely parallel alignment. The relative magnitude of the corner load error, i.e. the ratio between the error of the weight and the amount of the test weight being used corresponds approximately to the relative geometric deviation by which the error is caused. A distinction is made between a corner load error in the lengthwise direction and a corner load error in the transverse direction of the parallel-guiding mechanism, in accordance with the direction in which the test weight is shifted on the weighing pan in the corner load test of the balance. A corner load error in the lengthwise direction occurs when the vertical distance of the parallel guides at the end where they are connected to the stationary parallel leg is not exactly the same as at the opposite end where they are connected to the movable parallel leg. A corner load error in the transverse direction on the other hand occurs when the two parallel guides are twisted relative to each other, i.e. a condition where the distance between the parallel guides varies across the width of the parallel guides.
In the existing state of the art, for example, in EP 0 990 880 A2, in JP 2002 365125 A, and in WO 2005/031286, parallel-guiding mechanisms of weighing cells are disclosed which include a device for the adjustment of the corner load error. This adjustment mechanism follows a concept where the stationary parallel leg has at least one bending zone which is located between the fixation areas of the parallel guides and configured so as to define a tilt axis perpendicular to the lengthwise direction of the parallel-guiding mechanism. By tilting the fixation areas relative to each other by means of an adjustment screw, the end of the upper parallel guide that is connected to the stationary parallel leg can be raised as well as lowered. This allows the corner load error in the lengthwise direction to be corrected. Depending on the design of the adjustment mechanism, the pivot axis or the fixation area can be adjusted in its transverse tilt, whereby the corner load errors in the transverse direction of the weighing cell can be adjusted. In order to correct the corner load errors, i.e. to align the parallel guides so that they are parallel to each other, the fixation areas need to be tilted relative to each other only by a minute amount. Thus, the bending zones are subjected only to elastic deformation. This is important also for the reason that the bending zones are not overstressed under operating loads and therefore not subjected to plastic deformation. A plastic deformation of the bending zone would lead to a permanent out-of-parallel setting of the parallel guides and would have a detrimental effect on the weighing signal. Furthermore, the bending stress resulting from the elastic deformation and thus the restoring force of the bending zone is often used to clamp and thereby secure the adjustment screw.
All of the adjustment devices of the known state of the art with adjustment screws thus have in common that the adjusted position is maintained by means of the adjustment screws, and that the material in the bending zone is therefore in a permanent state of stress. It is therefore possible that over the long term an age-related stress could occur in this kind of corner load adjustment device due to relaxation of the material domains that are elastically stress-biased in one or the other direction. Reversible short-term changes can be caused by temperature fluctuation if the stationary parallel leg and the adjustment screws have different coefficients of thermal expansion.
As a countermeasure against the problem just described, a further adjustment possibility which does not require adjustment screws has been disclosed in a state-of-the-art reference. For example in weighing cells in which the weighing pan is guided by a monolithically configured parallel-guiding mechanism, as disclosed in commonly-owned U.S. Pat. No. 6,232,567 B1 to Bonino, the parallelism deviations of the parallel guides and, consequently, the corner load errors associated with them can be corrected by removing material from the bending zones of the parallel guides by grinding or filing. A removal of material from the topside causes the effective center of rotation of the flexure pivot to be offset in the downward direction, while a removal of material from the underside of the bending zone will offset its effective center of rotation in the upward direction.
The adjustment of corner load errors by removing material from the flexure pivots presents a problem in weighing cells which are designed for precision balances and analytical balances, i.e. for small weighing loads and high resolutions, and which therefore have slender flexure pivots. The grinding or filing to move material from a thin flexure pivot requires a sensitive touch. This operation is therefore in most cases performed manually and is accordingly cost-intensive.
In view of these unsatisfactory aspects of the corner load adjustment in parallel-guiding mechanisms of the currently known state-of-the-art, it is an object to provide a means of adjustment for the corner load errors in a parallel-guiding mechanism which avoids the aforementioned drawbacks and which can be realized in a simple manner at a favorable cost.